Several fixes for units errors:

Several fixes for units errors:
1) Do not assume that the superblock will be of size fs->fs_bsize.
   This fixes a panic when taking a snapshot on a filesystem with
   a block size bigger than 8K.
2) Properly calculate the number of fragments that follow the
   superblock summary information. This fixes a bug with inconsistent
   snapshots.
3) When cleaning up a snapshot that is about to be removed, properly
   calculate the number of blocks that need to be checked. This fixes
   a bug that created partially allocated inodes.
4) When moving blocks from a snapshot that is about to be removed
   to another snapshot, properly account for the reduced number of
   blocks in the snapshot from which they are taken. This fixes a
   bug in which the number of blocks released from a snapshot did not
   match the number that it claimed to have.